The invention pertains to corrosion to detection and particularly detection of metal corrosion in aircraft structure joints. More particularly, the invention pertains to sensors of metal corrosion in aircraft structure joints.
The applicants have recently devoted significant research efforts to understand the effects of corrosion in commercial transport and military aircraft. They have learned that nondestructive inspection of aircraft for corrosion involves an additional dimension of complexity beyond conventional non-destructive evaluation (NDE) inspection techniques for structural integrity problems such as fatigue cracking. Fatigue crack inspection is typically localized to joints and high stress locations on the fuselage structure. Hidden corrosion effects can occur, not only at such locations but throughout the aircraft structure. FIG. 1 shows some of the types of corrosion that occur on aircraft. Corrosions 11, 12, 13, 14, 15 and 16 are examples of pitting, inter granular exfoliation, stress corrosion cracking, inter granular cracking, crevice and galvanic corrosion and uniform microbial corrosion, respectively.
Corrosion occurs in areas of the fuselage subject to excess moisture or wetted by other fluids. These areas include the bilge of the fuselage for transport aircraft around wing fastener holes (primarily exfoliation), fuel shelf areas, wheel well shelves/backwalls in various aircraft, all doors including cargo access and landing gear doors.
Several existing non-destructive inspection (NDI) methods are available to detect corrosion, but each method is optimized to detect a particular type of corrosion. these detection methods include visual, tap test, electrical resistance probing, electrochemical analysis, ultrasonic, eddy current, X-ray radiography, and acoustic emission with heat. Visual inspection is appropriate for checking surface conditions such as pitting or exfoliation (blistering) but does little to detect hidden corrosion between lap joints. Acoustic emission NDE (e.g., the acoustic detection of hydrogen bubbles or gas) can detect material loss gaps between lap joints and structural cracks that could cause loss of structural integrity, but does not directly quantify the percentage of material loss or corrosion products.
The present invention makes it possible to detect hidden corrosion effects in aircraft structures. A key technical requirement is the detection of corrosion within the aircraft structure just below the head of an aircraft fastener, between lap joints or on the aircraft fuselage inner wall. The smart fastener concept focuses on the novel idea of integrating an electrochemical-based sensor directly into the aircraft structure to measure the evidence of active corrosion as an in-situ measurement without reducing aircraft structural integrity.